According to RP-060844 (“Proposed WID for Higher Order Modulation in HSUPA) of the 3rd Generation Partnership Project (3GPP), HSPA+ will add a new high order modulation-16-order Quadrature Amplitude Modulation (16QAM) in E-DCH.
As shown in FIG. 1, 16QAM will be equivalently implemented by multiplexing a pair of the 4 Pulse-Amplitude Modulation (4PAM) on the quadrature phase (Q branch) and the 4PAM oN the in-phase (I branch) according to the R1-070590 (“DRAFT Introduction of 16QAM for HSUPA”) of 3GPP.
As shown in FIG. 2, according to the section 5.2.1.1 of 3GPP TS 25.211 V7.0.0, the user equipment (UE) should specify the number of the bits of dedicated pilot, Transport Format Combination Indicator (TFCI), Feed Back Information (FBI), Transmission Power Control (TPC) of Dedicated Physical Control Channel (DPCCH).
As shown in FIG. 3, if there is no simultaneous Dedicated Physical Data Channel (DPDCH) and FBI when the UE using the E-DCH, DPCCH will use the transmission mode of “8 Pilot bits and 2 TPC bits” (in the following description, it is the transmission mode of “8 Pilot bits and 2 TPC bits” used unless otherwise specified.)
According to the section 5.1.2 of 3GPP TS 25.214 V7.3.0, the UE controls the transmission power of DPCCH, for example, the method of 1500 Hz power control or the method of the 5 time slots combined 300 Hz power control is performed at each time slot. According to section 5.1.2.5B.2 of 3GPP TS 25.214 V7.3.0, the UE controls the transmission power of Enhanced Dedicated Physical Data Channel (E-DPDCH), and the method would be to set the power offset between E-DPDCH and DPCCH.
Suppose the modulation of E-DPDCH in a certain 2 ms-Transmission Time Interval (TTI) is Binary Phase Shift Keying (BPSK), the power of E-DPDCH in the 2 ms-TTI might be different in each time slot, for example, if the 1500 Hz fast power control were applied, each control should use 1 dB for control granularity.
Suppose the modulation scheme of E-DPDCH in a certain 2 ms-Transmission Time Interval (TTI) is 4PAM (I/Q multiplexed to be equivalent to 16QAM), the power of E-DPDCH in the 2 ms-TTI should be as constant as possible so as to make the received data reliable. The advantage of keeping the power of E-DPDCH constant is that, the channel estimation at a base station (Node B) in fields except pilot of DPCCH could fully make use of the channel estimation in the pilot field of the next time slot, as shown in FIG. 4.
On the contrary, if the transmission power in each time slot of the 2 ms-TTI is variable (increasing or decreasing a power control order) when 4PAM data (I/Q multiplexed to be equivalent to 16QAM) is transmitted in E-DPDCH, an error occurs easily in channel estimation during TPC time, and the channel estimation in the whole TTI is inconsistent, as shown in FIG. 5.
In a practical channel condition, due to multipath, Doppler shift, or interference due to other radio devices, the originally orthogonal I branch signal and Q branch signal sent by the UE are not orthogonal anymore at receiver of Node B, and there is crosstalk.
For example, suppose the transmitted signal is shown as FIG. 6, the signal received in the practical channel condition is shown as FIG. 7, wherein the received I branch signal is shown as FIG. 8, and the received Q branch signal is shown as FIG. 9. From FIG. 7 to FIG. 9, it can be seen that there is symbol misjudgment at the receiving terminal.